Combine stream of Collections into one Collection – Java 8

This functionality can be achieved with a call to the flatMap method on the stream, which takes a Function that maps the Stream item to another Stream on which you can collect.

Here, the flatMap method converts the Stream<Collection<Long>> to a Stream<Long>, and collect collects them into a Collection<Long>.

Collection<Long> longs = streamOfCollections
    .flatMap( coll -> coll.stream())
    .collect(Collectors.toList());